Multilayered printed circuit boards are being produced mainly by (a) a mass laminating technique and (b) a pin laminating technique. In these techniques, a printed circuit board for inner layer use (hereinafter, referred to as "inner-layer board") is first manufactured, this inner-layer board is combined with prepregs and either a copper foil or a single-side copper-clad laminate, and the superposed laminating materials are laminated to give a multilayered board both sides of which are constituted by a copper foil. This multilayered board is subjected to processing steps such as steps for forming through-holes, outer-layer printed circuits, etc.
As expedients for enabling inner-layer boards to have improved adhesive strength in multilayered printed boards, it is known that a copper foil both sides of which have been roughened beforehand is used, and that surface-roughening treatment of copper foil is conducted after an inner-layer board is manufactured and a printed circuit is formed thereon. Known techniques for the surface roughening treatment in the latter expedient include (1) a method in which the copper foil surface is treated with an oxidation-treated aqueous solution to form a finely roughened copper-oxide layer on the surface, (2) a method in which the copper foil surface is treated with a silane coupling agent, organotitanate coupling agent, or the like, and similar methods. Conventionally, however, inner-layer boards obtained by forming inner-layer printed circuits on copper-clad laminates having a glossy surface and then treating the copper foil surfaces with an oxidizing aqueous alkaline solution to form a finely roughened layer constituted by a brown to black copper oxide have usually been used from the standpoints of adhesiveness and economy.
However, this finely roughened layer constituted by a copper oxide, particularly copper(II) oxide is brittle and tends to dissolve in acidic aqueous solutions such as hydrochloric acid, sulfuric acid, and the like. Because of this, there has been a problem that in the case where multilayered boards produced by laminating using such copper foils are processed to form through-holes therein and then subjected to through-hole plating, electroless plating, or subsequent electroplating, the acidic solution gradually penetrates into the finely roughened copper-oxide layer from the layer's exposed parts in the through-hole walls and attacks the copper oxide layer to cause so-called "harrow" or "pink ring", which may impair insulating and other properties, resulting in final printed circuit boards with poor reliability.
As an expedient for preventing harrow or pink ring which is developed in multilayered boards using the copper foil having a finely roughened surface layer constituted by a brown to black copper oxide, (3) a method was recently disclosed in which the copper oxide constituting the finely roughened layer on the surface of the copper foil treated by method (1) above is reduced with an aqueous solution of a reducing agent, thereby to convert the copper oxide into cuprous oxide or copper metal (e.g., JP-A-56-153797). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Although the copper foil treated by this method can still have practically acceptable adhesive strength, this method newly necessitates a liquid-phase reduction treatment step. Further, since the adhesive strength of the reduction-treated copper foil is lower than that of the copper foil having the finely roughened surface layer constituted by a brown to black copper oxide, it is necessary that the reduction conditions should be strictly regulated in order to obtain practically acceptable adhesive strength. In addition, the above method has been also defective in that the effect of preventing occurrence of "harrow" is not stably exhibited and the method has newly posed problems concerning the treatment of waste liquids resulting from the use of aqueous reducing agent solutions.